Parasitism is a form of living in which two organisms that are phylogenically different (unrelated) co-exist over a long period of time (usually for the lifetime of one of the individuals). Parasitism can also be classified as a specialized form of predation that may be lethal or non-lethal to one or both of the organisms.

There are several different outcomes of parasitism. When a parasite lives with another organism, this can cause commensalism which is defined as two organisms that co-exist in the same space/area/location, and one of the organisms benefit from the other while neither harming nor helping the other organism. Parasitism may also be related to mutualism which is non-lethal because both organisms benefit rather than one organism benefiting more than the other organism. An example of parasitism would be the parasite (which is usually a smaller organism) benefits from the other organism (the host). Here the host is usually harmed.

The term parasite has come to mean a eukaryotic, pathogenic organism. Some Parasites require the use of vectors, such as insects, that carry the parasite to a reservoir.

It is a fact that nearly half of all the animals have at least one parasitic phase in their life cycles. So basically, all free-living animals are host to one or more parasite taxa at some point in their lives.

Some examples of endoparasites include Giardia lamblia , an anaerobic protozoan parasite which reproduces via binary fission. It affects humans, cats, and dogs, among other wild animals. Another endoparasite is the hookworm, either Ancylostoma duodenale or Necator americanus, which infect humans.

Examples of ectoparasites include the Rocky mountain wood tick, Dermacentor andersoni, and a mosquito. Mistletoe, is an example of an epiparasite, called an aerial parasite. A plant parasite, is a parasite that attaches to the branches of trees or shrubs.

There are several different types of parasites. Parasites that live in the body are called endoparasites. An example of an endoparasite is a hookworm that lives in the hosts gut. Parasites that live on the outside of the body are called ectoparasites. Epiparasite is a parasite that feeds off another parasite, this is called hyperparasitoidism. Examples of this situation is a wasp or fly larva being an endoparasite in wasp larva and the wasp larva being an endoparasite of a wood boring beetle larva.

There are several ways endoparasites get access to the host’s body. Ways they gain access are through an organism’s tissue and raw foods. An example of a organism that gains access through raw foods is the nematode Ascaris lumbricoides. It gains access through the human intestine where it produces large numbers of eggs that are passed to the hosts digestive tract and pancreas in an external environment. They often get inadvertently digested by humans in places without good sanitation.

Generally parasites are thought of as potentially harmful organisms. Malaria, a protist, is considered a parasite, the mosquito is the vector, and the human is the host. There are a large number of plants that are considered parasites that people find attractive and useful; these plant parasites are known as epiphytes. One of the most common epiphytes is the orchid.

Orchids like most other epiphytes do not harm their host and do not use nutrients from the host. The epiphyte relies on nutrients it receives from the surrounding air and water from rainfall. Orchids and other epiphytes in the rainforests have a distinct advantage of not growing on the forest floor. Instead they grow on the sides of trees which allows for easier capture of sunlight with less interference from the jungle canopy. Orchids are the most diverse of flowering plants with over 18,000 species and represent about 8 percent of the flowering plant species worldwide. Recent estimations show that there are between 10,000 and 12,000 species of undiscovered orchids. Orchids tend to be endemic to microhabitats like valleys. With deforestation and land clearing it is estimated that hundreds of orchid species are lost each year.

An important role of epiphytes in the rainforests is creating niches for a wide range of species. One of the best observed microsystems based upon an epiphyte is the Tank Bromeliad of South America. This epiphyte can hold up to two gallons of water in its upturned leaves. This water is used as a drinking supply for canopy animals and also creates a habitat for breeding insects and animals.

Numerous mosses and lichens are also considered epiphytes and another common epiphyte at Christmas is Mistletoe. Mistletoe is one of the epiphytes that can be harmful to its host. The berries from mistletoe are covered with a sticky laxative coating and are spread by birds. These seeds develop haustorium when germination begins and penetrate the bark of young trees and lethal infections can subsequently occur.

Organisms called Brood parasites[[1]] use the strategy of brood parasitism which is often an overlooked type of parasitism. This involves the use of host individuals of the same or different species to raise the young of the brood parasite. This therefore leaves the parasite with no responsibility of rearing their young. This leaves them more time to focus on other things such as producing more offspring. The most common types of brood parasites are usually birds and insects. A couple of examples of birds are the Brown-Headed Cowbird, and the Goldeneye duck. Cuckoo bees and wasps are some examples of insects that are brood parasites.

The Cowbird family is the bird family in North America that is an obligate brood parasite(one that must parasitize because they are not capable of building a nest). While most brood parasites have eggs that mimic the hosts eggs the cowbird is again different. The cowbird has been documented to have used over 200 different host species.

This phenomenon usually occurs in wasps, where the parasitic association usually results in the death of the host. A wasp will lay her eggs in front of, on, or in another insect, sometimes a grub. The larvae hatch inside of the grub, feeding on it and growing, until they burst out of the organism, killing it in the process.

The most important aspect of the parasite's life is the selection of a host. Without a host species the parasite does not and can not survive. For a parasite to survive, the host is required to complete the life cycle for the parasite. Since this is a vital part of the parasite's livelihood, the parasite will rarely kill their host species. Unfortunately for both parties, the simple presence of a parasite can cause much harm to the host. The choice of a host species is observed through host specificity and host range.

A full-grown tapeworm that infects humans and lives in the small intestine

Host specificity is seen whenever a parasite is supported by only a single host. The host specificity of a parasite is very important to parasites that flourish only in certain environments and conditions. Many parasites in fact are limited to only one or a few closely related host species. Even though it is possible for a parasite to infect different types of species, they still tend to parasitize certain species more than others. An example of this occurrence can be seen in the parasitic flatworm Monogenea. The Monogenea have been known to be parasitic on teleosts, elasmobranchs, amphibians, and reptiles. However, the highest concentration of these flatworms has been seen on the teleosts(ray-finned fishes). The Monogenea show high host specificity for the teleosts.

The host range of a parasite is when that organism is supported by a number of host species. It is also a measurement of the range of host species a parasite will/can infect. An example of a parasite that has a wide range of host species is the common mosquito that not only parasitizes humans, but other animals as well. The mosquito requires the blood of other animals to sustain its life cycle. Host specificity and host range can be compared to the concepts introduced in Chapter 11, Ecology/Predation and Herbivory, that discusses monophagous and polyphagous herbivores that either eat one species of plant and are dependent on that food source or eat many species of plants to have a wider range of possible sustenance.

Some parasites can manipulate their host’s behavior; a good example of a parasite with this manipulation ability is the hairworm. Their lifecycle consist of a transition from an aquatic habitat, where they live off aquatic insect larvae, to a terrestrial habitat where they live off crickets and or grasshoppers. Studies of crickets infested with mature hairworms have shown that the crickets are more likely to seek water. This is when the hairworms have reached their free living stage. Once the crickets reach the water the hairworms emerge and cause the crickets to struggle about the water. This process causes an increase of predation by fish to prey on the struggling crickets [2].

Hosts have evolved various adaptations to combat parasites initial infection and proliferation. These adaptations can be as complex as antigen specific cell mediated responses which are most common in vertebrates or simpler like barrier mechanisms. Hosts also have behavior defenses that reduce the chances of an individual coming in contact with a parasite. Parasites can have an effect on host life-history evolution, sexual selection and also population dynamics [3].

There are two kinds of host immunities; adaptive immunity, whereby the host produces antibodies which are parasite-specific, and cellular immunity, which are non-specific. Some techniques a host may utilize to defend against parasites include evasion, preening, grooming, and self medication.

Evasion is one of the first immune defense host response against infection as it imposes several barriers to viral replication and spread [4]. The most common type of evasion is when we have a viral or bacterial infection and your mucus glands work to excrete the infection through the nearest exit, such as the nose. Another interesting example of evasion can be found in Infection and Immunity Journal at:[[5]]

Grooming is a very common way for hosts to control parasites. An example would be primates sitting together picking them off each other and eating them. Self-medication could be a possibility also, a host could eat a type of plant that may force an evasion.

Certain ectoparasites decrease the reproductive success of breeding birds, and example of one of these ectoparasites is Blow flies Protocalliphora spp., which cause a direct loss of nutrients to the nestlings. Some nestling birds use dietary nutrients such as carotenoids to lessen the effects that parasites have on them [6].

The effect of a parasite on the host population can cause an epidemic (outbreak) of a disease in a community. Some diseases may show cycles similar to predator-prey cycles. This shows the struggle that occurs between parasites and there host. In humans, whooping cough and measles are examples of diseases where parasites infect host. Host that have never been infected are susceptible to a disease the first time, but after that they are non-susceptible to the infectious host. After an outbreak enough of the hosts in the community become immune to the disease and the disease will decline in the population. This will cause the epidemic to be wiped out and it will not return until there are a number of new individuals born or new immigrants brought into the community.

A mosquito is a parasite that can infect its host with a disease.

Disease Model: Rp=SBL

Rp: The replacement rate of infected hosts.

S: The density of the susceptible host.

B: Transmission Rate

L: The average period during which a host will be infectious.

When Rp has a value that is equal to or greater than 1 the outbreak will return in the community. This will start the cycle over again since once there are more non-susceptible hosts the outbreak will die off again. When the disease affects children the cycle will happen very rapidly do to the high turnover rate. When a disease is fatal to an individual host population, the host can be limited and controlled. This means that something as small as a parasite can control the population of large animals.

Parasites can also have an affect on competition between two species. There are some species that compete for food and resources, and parasites can give some species an advantage. One example is white tail deer and there immunity to Parelaphostrongylus tenuis. While the white tail deer are immune to the disease moose, mule deer, and pronghorns are not immune. This means that the population of these species can be controlled by the parasite that has infected them. Probably the most well known example of parasitism is malaria which is caused by the mosquito.

Phoresis is a defined behavior in which less mobile parasite species hitchhike on other, more mobile parasite species. An example of such behavior can be seen in the interaction between feather-feeding lice and louse flies. However, it should be noted that not all species of feather-feeding lice engage in such behvior. Feather lice (Phthiraptera: Ischnocera) are wingless. They are also permanent parasitisers of birds. Transmission to new hosts is made possible by catching a ride (hitchhiking) on parasitic louse flies. Louse flies do have wings and fly between individual hosts. Because every host eventually dies, phoresis, an evolved mode of transmission, is essential for the pesistence of many parasite lineages [7].

The specific blocking of host reproduction by a parasite is known as parasitic castration. This host-parasite interaction is intensity-independent, meaning a single parasite can bring about the blocking of the host's reproductive capability. Parasites benefit from parasitic castrations by diverting the energy a host puts forth in reproduction into their own growth and reproduction. The most studied parasitic castration systems involve invertebrates, particularly crustaceans, echinoderms, and molluscs. Parasitic castrations of vertebate hosts have been seldomly recognized. In a recent study, it has been demonstrated that a common external cymothoid isopod, Anilocra apogonae, castrates Indo-Pacific reef fish host Cheilodipterus quinquelineatus[8].